In portable devices such as notebook PCs and portable telephones, batteries for low power consumption of about 5V DC (volts direct current) are used as power supplies so that the devices can be used for extended periods even at locations where a commercial power supply cannot be used. In these portable devices, however, there are circuit modules that operate at an AC (alternating current) potential of about +/−100V with a special potential waveform, e.g., vibrating elements that indicate to the user that an input operation has been acknowledged or that a signal has been received.
Conventionally, operating this type of circuit module requires a potential step-up circuit that raises a low potential of approximately 5V to a potential of approximately +/−100V and a wave-shaping circuit that shapes the DC potential waveform obtained from the power supply into a desired drive potential waveform suited for the circuit module (see, e.g., sections 0081–0085 and FIG. 7 of Japanese Laid-Open Patent Publication No. 2004-21697).
FIG. 10 shows a conventional wave-shaping circuit 100 that shapes an output potential to vibrate a piezoelectric substrate 13 serving as the circuit module. In the wave-shaping circuit 100, a step-up oscillator circuit 101 generates an oscillation of 20–200 kilohertz (kHz) for the fixed DC potential power supply of a few volts. A potential step-up circuit 102 switches the current flowing through a coil at a period based on the step-up oscillator circuit 101. The fixed DC potential power supply of a few volts is increased to a DC potential of 100V, which is sent to an amplifier circuit 103.
A vibration oscillator circuit 104 generates a drive signal at a frequency that operates a piezoelectric substrate 13, and this drive signal is sent to the amplifier circuit 103. The amplifier circuit 103 amplifies the drive signal using the DC potential received from the step-up circuit 102, and the result is sent to the piezoelectric substrate 13 to serve as the drive potential.
With this wave-shaping circuit 100, a drive potential waveform suited for the operation of the circuit module (piezoelectric substrate 13) can be generated by setting up the frequency of the drive signal generated by the vibration oscillator circuit 104 and the pulse width generated by a pulse width generation circuit 106.
However, this conventional wave-shaping circuit 100 is equipped with circuits having two objectives: stepping up the voltage with the step-up circuit 101 and obtaining the desired potential waveform with the circuits 104, 105, 106. Thus, large circuit components, including transformers, are necessary, resulting in a complex circuit structure, a higher number of components, and increased costs. In particular, with portable devices that need to be carried around, there is a need to make the device as light and compact as possible. The inclusion of large components such as a transformer makes it impossible to meet this need.
Furthermore, a high potential is continuously generated by the step-up circuit 102 regardless of whether or not there is an incoming vibration trigger. This increases the danger of electrical leakage and short-circuits. Furthermore, since power is continuously consumed to step up the voltage, the lifespan of the battery is reduced.